1. Field of the Invention
The present invention relates to a control circuit. Further, the present invention relates to a DCDC converter controlled by the control circuit and a driving method thereof.
2. Description of the Related Art
PWM control is known as one of methods for controlling a DCDC converter that changes output power in accordance with the load of a circuit and outputs stable voltage.
Pulse width modulation (PWM) is a modulation method of changing the duty cycle of an output pulse. For example, in the case of applying PWM to a DCDC converter, an output voltage can be close to a desired value in the following manner: the difference between an intended voltage and an output voltage that varies depending on the size of a load of a circuit connected to the output side of the DCDC converter is fed back, and the duty cycle of an output pulse is changed in accordance with the difference.
As an example of a conventional DCDC converter, FIG. 6 illustrates an example of a step-up DCDC converter to which a control circuit is connected. A DCDC converter 10 includes a control circuit 20, a switching transistor 11, an inductor 13, a diode 15, a capacitor 17, a resistor 19a, and a resistor 19b. Moreover, the DCDC converter 10 has an input terminal (POWER) to which a DC power source is connected, and an output terminal (OUTPUT) connected to a load circuit.
An output terminal of the control circuit 20 is connected to a gate of the switching transistor 11. The on/off state of the switching transistor 11 is controlled with a pulse output from the control circuit 20.
When the switching transistor 11 is on, a current flows through the inductor 13 in accordance with the difference between an input voltage and a ground voltage. Since electromotive force is generated by self induction in the direction opposite to that of the current flowing through the inductor 13, the current is gradually increased.
Next, when the switching transistor 11 is turned off, the path of the current that has passed through the inductor 13 until then is interrupted. In the inductor 13, electromotive force is generated in the direction that interferes with the change of this current, that is, in the direction opposite to that at the time when the switching transistor 11 is on. Thus, a current corresponding to this electromotive force flows through the inductor 13. At this time, the potential of a node between the inductor 13 and the diode 15 is higher than the potential of the input terminal, so that a voltage higher than an input voltage (a power supply voltage of the DC power source) is output to the output terminal of the DCDC converter 10. The converter with this configuration is therefore called a step-up converter.
The potential of the node between the inductor 13 and the diode 15 at the time when the switching transistor 11 is off is increased in proportion to the current that has flowed through the inductor 13 just before the switching transistor 11 is turned off. In other words, the longer the time during which the switching transistor 11 is on is, the higher the potential of the node is. Thus, when the duty cycle of an output pulse signal of the control circuit 20 is high, the voltage can be stepped up so that the difference between the output voltage and the input voltage is large, whereas when the duty cycle is small, the voltage can be stepped up so that the difference of these voltages is small. By adjusting the duty cycle, the output voltage of the DCDC converter 10 can be closed to a desired voltage.
Here, the control circuit 20 includes a triangle-wave generator circuit 21, an error amplifier 23, and a PWM buffer 25. The error amplifier 23 outputs a voltage signal whose level corresponds to the difference between the output voltage of the DCDC converter 10 and a desired voltage. The PWM buffer 25 outputs a pulse having a waveform with a duty cycle corresponding to the above voltage difference to the gate of the switching transistor 11, by using an output voltage from the error amplifier 23 and a triangle-wave voltage output from the triangle-wave generator circuit 21.
FIG. 7 is a schematic diagram of two input signals input from the triangle-wave generator circuit 21 and the error amplifier 23 to the PWM buffer 25, and an output signal of the PWM buffer 25 generated with the input signals. Here, a solid line 51 indicates the input signal input from the triangle-wave generator circuit 21 to the PWM buffer 25. A solid line 53 indicates the input signal input from the error amplifier 23. A solid line 55 indicates the output signal of the PWM buffer 25. The PWM buffer 25 in this configuration compares a voltage of the input signal from the error amplifier 23 and a voltage of the input signal from the triangle-wave generator circuit 21, and outputs a high-level voltage when the voltage of the input signal from the triangle-wave generator circuit 21 is the higher and outputs a low-level voltage when the voltage of the input signal from the triangle-wave generator circuit 21 is the lower. As illustrated in FIG. 7, the duty cycle of the output pulse from the PWM buffer 25 is changed in accordance with the level of the voltage of the input signal from the error amplifier 23, that is, in accordance with the difference between the output voltage of the DCDC converter 10 and a desired voltage.
In general, when considering the power efficiency of a DCDC converter, power loss due to components other than a control circuit (e.g., parasitic resistance of an inductor and a capacitor, voltage across a diode, and on-resistance of a switching transistor) is a dominant factor on the high output power side with a large load current. On the other hand, on the low output power side with a small load current, power loss due to the control circuit is a dominant factor in degradation of power efficiency.
When considering factors in power loss due to the control circuit, power loss due to a PWM buffer is the largest among three circuit elements of a triangle-wave generator circuit, an error amplifier, and the PWM buffer. Specifically, power loss due to the influence of charging and discharging of gate capacitance of a switching transistor connected to the PWM buffer is given. The power loss at the time of switching is proportional to the value of the gate capacitance of the switching transistor and proportional to switching cycles per unit time, that is, the sampling frequency. Here, the sampling frequency is a frequency equal to the oscillation frequency of a triangle wave output from the triangle-wave generator circuit.
Therefore, the sampling frequency needs to be lowered in order to increase the power efficiency on the low output power side of a DCDC converter. In light of the above, a method of changing the oscillation frequency of a triangle-wave generator circuit has been proposed (Patent Document 1).